Oligo Ethylene Glycol Chains Research Articles

Light-regulated crystal growth of \u03c0-conjugated luminophores in an azobenzene matrix

Control over the phase transition of functional molecules is a key to design stimuli-responsive materials. Although many efforts have been devoted toward controlling the phase transition of functional molecules by various stimuli such as temperature, solvent vapor, and mechanical stimuli, indirect control using other stimuli-responsive molecules has been hardly explored. Here we demonstrate the potential of this methodology by using a luminescent and a photoresponsive molecule. We prepare blend films composed of an oligo(p-phenylenevinylene) amphiphile showing intrinsic luminescent chromism through isotropic-to-crystalline phase transition and a photo-liquefiable azobenzene amphiphile. The two materials are designed to co-assemble on the molecular level by introducing identical alkyl and oligo(ethylene glycol) chains. The blend films exhibit a luminescence color change from orange to green upon rubbing and subsequent exposure to UV light. Structural analyses reveal that the crystallization of the luminescent amphiphile is regulated by the photoinduced isotropization of the azobenzene amphiphile.

Using Alkali Metal Ions To Template the Synthesis of Interlocked Molecules.

In 1987, Pedersen, Cram, and Lehn were awarded the Nobel Prize in Chemistry to honor their achievements in, among other things, the selective recognition of alkali metal ions by synthetic hosts. Almost three decades later, the 2016 Nobel Prize went to Stoddart, Sauvage, and Feringa for the development of artificial molecular machines, in which interlocked molecules play a significant role. Surprisingly, although many rotaxane- and catenane-based molecular machines have been constructed using various templating approaches, alkali metal ions, which are good templates for crown ether synthesis, have only rarely been applied as templates for the assembly of these interlocked molecules. This paucity of examples is probably due to the less well defined coordination numbers and geometries in the complexation of alkali metal ions to common oxygen-containing ligands, resulting in much weaker metal-ligand interactions and less predictable structures for their complexes compared with those formed between transition metal ions and common pyridine-containing ligands. Nevertheless, the ease of removing alkali metal ions from interlocked compounds and their much lower toxicity compared with that of transition metal ions are attractive features that have inspired their use as templates in the synthesis of interlocked molecules. About a decade ago, we began investigating the feasibility of using alkali metal ions to template the formation of catenanes and rotaxanes, with the hope of developing facile, broadly applicable, green, and efficient methods for their construction. We noticed that the interactions between oxygen-containing ligands and alkali metal ions can be strengthened by minimizing the effects of competing interactions from solvent molecules and counteranions. Thus, to increase the solubility of the metal ion salts in less polar solvents (e.g., CH2Cl2, CHCl3) and minimize ion pairing, we chose tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB), a weakly coordinating anion, as the counteranion for the alkali metal ions applied as templates. Our strategy has been based on the association of simple and general recognition units: (i) the orthogonal arrangement of two oligo(ethylene glycol) chains around an alkali metal ion and (ii) the encircling of a single urea/amide unit by an oligo(ethylene glycol)-containing macrocycle in the presence of a templating alkali metal ion. The former recognition system has allowed the facile construction of many interesting interlocked structures, including cyclic [2]catenane trimers and tetramers; the latter has provided several rotaxanes, including some incorporating monomers of practically important (macro)molecules (e.g., peptides, polymers) and some that behave as switches with unique functions (e.g., catalysis, gelation). The components in these recognition systems possess high flexibility in terms of their structures and the choice of suitable alkali metal ion templates. This Account tells the story of the concept behind this alkali metal ion-templating approach as well as its elaboration, scope, and recent advances. We hope to convince the reader that alkali metal ions are powerful templates for assembling interlocked structures and compounds and also to demonstrate the range of possibilities that they provide for future endeavors.

Grafting of Oligo(ethylene glycol)-Functionalized Calix[4]arene-Tetradiazonium Salts for Antifouling Germanium and Gold Surfaces.

Biosensors that can determine protein concentration and structure are highly desired for biomedical applications. For the development of such biosensors, the use of Fourier transform infrared (FTIR) spectroscopy with the attenuated internal total reflection (ATR) configuration is particularly attractive, but it requires appropriate surface functionalization of the ATR optical element. Indeed, the surface has to specifically interact with a target protein in close contact with the optical element and must display antifouling properties to prevent nonspecific adsorption of other proteins. Here, we report robust monolayers of calix[4]arenes bearing oligo(ethylene glycol) (oEG) chains, which were grafted on germanium and gold surfaces via their tetradiazonium salts. The formation of monolayers of oEGylated calix[4]arenes was confirmed by AFM, IR, and contact angle measurements. The antifouling properties of these modified surfaces were studied by ATR-FTIR spectroscopy and fluorescence microscopy, and the nonspecific absorption of bovine serum albumin was found to be reduced by 85% compared to that of unmodified germanium. In other words, the organic coating by oEGylated calix[4]arenes provides remarkable antifouling properties, opening the way for the design of germanium- or gold-based biosensors.

From Foldable Open Chains to Shape-Persistent Macrocycles: Synthesis, Impact on 2D Ordering, and Stimulated Self-Assembly.

Small molecule self-assembly at surfaces offers an efficient route to highly ordered organic films that can be programmed for a variety of chemical and electronic applications. The success of these materials depends on the ability to program intermolecular interactions to guide precise structural ordering. Toward this objective, we have designed and synthesized a series of bis(triazolo)benzene-based π-conjugated molecules. Our synthesis exploits a last-stage C-C cross-coupling reaction to close up zigzag-shaped linear precursors to cyclized products, so that direct side-by-side comparisons can be made for their structure-dependent self-assembly behavior at surfaces and response to external stimuli. Indeed, scanning tunneling microscopy (STM) analysis revealed distinct differences as the conformational flexibility of the molecular backbone and the chemical structure of the peripheral groups are varied. Specifically, alkyl chains adsorb and form interdigitated structures, whereas oligo ethylene glycol (OEG) chains remain desorbed and thus shift self-assembly to more densely packed π-conjugated cores. While the macrocycles self-assemble immediately and spontaneously, their linear precursors exhibit slower self-assembly kinetics, which could be attributed to the difference in the degree of conformational freedom. We also found that perturbation by the STM tip and the addition of cosolutes profoundly impacted the kinetics of self-assembly and surface patterning. This highly unusual behavior highlights the importance of noncovalent interactions that are inherently weak in solution but can be made strong for symmetric and conformationally restricted molecules confined within 2D surfaces.

A High Dielectric N‐Type Small Molecular Acceptor Containing Oligoethyleneglycol Side‐Chains for Organic Solar Cells

We report a new small molecular acceptor, ITIC‐OEG, which is based on indacenodithieno[3,2‐b]thiophene and 1,1‐(dicyanomethylene)‐3‐ indanone including oligoethyleneglycol (OEG) side‐chains. ITIC‐OEG was found to have higher dielectric constant (εr=5.6) than that of a reference molecule of ITIC with normal alkyl substituents (εr=3.9). The dielectric constant of medium influences significantly the exciton binding energy and the resulting charge separation and recombination. The optical, electrochemical and morphological properties of ITIC‐OEG and its photovoltaic characteristics were investigated by blending with a semi‐crystalline donor polymer, PPDT2FBT, with comparison to those of ITIC. ITIC‐OEG shows more red‐shifted absorption and stronger crystalline packing than ITIC. However, the lower photovoltaic performance (with 1.58% power conversion efficiency, PCE) was measured for PPDT2FBT:ITIC‐OEG, compared to PPDT2FBT:ITIC (5.52% PCE). The incompatibility between PPDT2FBT and ITIC‐OEG (due to high hydrophilic nature of OEG chains) resulted in poor intermixing with large domain separation over 300 nm, showing inefficient charge separation and significant charge recombination. Therefore, to investigate the effect of dielectric constant of the materials on the charge separation and recombination, the blend morphology of the PPDT2FBT:ITIC‐OEG should be optimized first by improving their miscibility and phase separation.

Kinetically Controlled Stepwise Self-Assembly of AuI-Metallopeptides in Water.

The combination of attractive supramolecular interactions of a hydrophobic AuI-metallopeptide with the shielding effect of flexible oligoethylene glycol chains provides access to a stepwise self-assembly of a AuI-metalloamphiphile in water. Kinetic control of the supramolecular polymer morphology is achieved using a temperature-dependent assembly protocol, which yields low dispersity supramolecular polymers (metastable state I) or helical bundled nanorods (state II).

Saccharide Recognition and Helix Formation in Water with an Amphiphilic Pyridine–Phenol Alternating Oligomer

An amphiphilic pyridine–phenol alternating oligomer, in which oligo(ethylene glycol) chains are attached at the 4‐positions of the pyridine and the phenol rings, was synthesized as a helical host to realize saccharide recognition in water. This oligomer was soluble not only in less‐polar solvents but also in water at room temperature. Circular dichroism investigations revealed that the oligomer could recognize several kinds of saccharides and their derivatives, including glycodrugs to form chiral higher‐order structures in water. Especially, the oligomer showed high affinity for d‐glucosamine, and their association constants were over 103 m–1.

Ionophores at work: Exploring the interaction of guanosine-based amphiphiles with phospholipid membranes

An amphiphilic derivative of guanosine, carrying a myristoyl group at the 5′-position and two methoxy(triethylene glycol) appendages at the 2′ and 3′-positions (1), endowed with high ionophoric activity, has been here studied in its interaction mode with a model lipid membrane along with its 5′-spin-labelled analogue 2, bearing the 5-doxyl-stearic in lieu of the myristic residue. Electron spin resonance spectra, carried out on the spin-labelled nucleolipid 2 in mixture with a DOPC/DOPG phospholipid bilayer, on one side, and on spin-labelled lipids mixed with 1, on the other, integrated with dynamic light scattering and neutron reflectivity measurements, allowed getting an in-depth picture of the effect of the ionophores on membrane structure, relevant to clarify the ion transport mechanism through lipid bilayers. Particularly, dehydration of lipid headgroups and lowering of both the local polarity and acyl chains order across the bilayer, due to the insertion of the oligo(ethylene glycol) chains in the bilayer hydrophobic core, have been found to be the main effects of the amphiphilic guanosines interaction with the membrane. These results furnish directions to rationally implement future ionophores design.

Controlled Functionalization of Gold Nanoparticles with Mixtures of Calix[4]arenes Revealed by Infrared Spectroscopy.

Labile ligands such as thiols and carboxylates are commonly used to functionalize AuNPs, though little control over the composition is possible when mixtures of ligands are used. It was shown recently that robustly functionalized AuNPs can be obtained through the reductive grafting of calix[4]arenes bearing diazonium groups on the large rim. Here, we report a calix[4]arene-tetradiazonium decorated by four oligo(ethylene glycol) chains on the small rim, which upon grafting gave AuNPs with excellent stability thanks to the C-Au bonds. Mixtures of this calixarene and one with four carboxylate groups were grafted on AuNPs. The resulting particles were analyzed by infrared spectroscopy, which revealed that the composition of the ligand shell clearly reflected the ratio of calixarenes that was present in solution. This strategy opens the way to robustly protected AuNPs with well-defined numbers of functional or postfunctionalizable groups.

Assembly of [2]Rotaxanes in Water

Two [2]rotaxanes have been assembled in water from modular subunits through CuI‐catalyzed azide–alkyne “click” chemistry. For this purpose, 2,6‐disubstituted naphthalene axles with solubilizing oligo(ethylene glycol) (OEG) chains (n = 1–5) and propargyl terminal groups were synthesized and examined for their propensity to form inclusion complexes with a dicationic Diederich‐type cyclophane host. The dependence of pseudorotaxane formation on the linkers between the naphthalene core and OEG chains, and in the case of ester linkers on different spacer lengths, was analyzed by titration experiments. In addition, the inclusion complexes of two [2]rotaxanes were trapped by using a water‐soluble azide‐functionalized stopper. Repetitive chromatography finally enabled the isolation of both mechanically interlocked [2]rotaxanes.

Pt⋯Pt and π–π interactions-induced pleated polymeric foldamers

Two phenylethynylplatinum(II) terpyridine (PEPtTPY)-derived polymers PtP1 and PtP2 have been prepared from the coupling reactions of two PEPtTPY-incorporated dialdehydes and an oligo(ethylene glycol)-derived diacylhydrazine which form hydrazone bonds quantitatively. Phosphorescence and absorption studies revealed that in binary MeCN and DMSO the PEPtTPY units of both polymers undergo intramolecular Pt⋯Pt interaction and π–π stacking to induce the backbones of the polymers to form pleated conformations. Adding LiClO4 or NaClO4 further stabilizes the two polymeric pleated foldamers by electrostatically binding the oligo(ethylene glycol) chains to induce the chains to fold.

Use of Membrane Potential to Achieve Transmembrane Modification with an Artificial Receptor.

We developed a strategy to modify cell membranes with an artificial transmembrane receptor. Coulomb force on the receptor, caused by the membrane potential, was used to achieve membrane penetration. A hydrophobically modified cationic peptide was used as a membrane potential sensitive region that was connected to biotin through a transmembrane oligoethylene glycol (OEG) chain. This artificial receptor gradually disappeared from the cell membrane via penetration despite the presence of a hydrophilic OEG chain. However, when the receptor was bound to streptavidin (SA), it remained on the cell membrane because of the large and hydrophilic nature of SA.

Au-Mesoporous silica nanoparticles gated with disulfide-linked oligo(ethylene glycol) chains for tunable cargo delivery mediated by an integrated enzymatic control unit.

We report a delivery system based on Au-mesoporous silica (MS) nanoparticles functionalized with acetylcholinesterase on the Au face as a control unit and with disulfide-containing oligo(ethylene glycol) chains on the MS face as caps. The control unit handles the chemical information in the environment (the presence of acetylthiocholine or enzyme inhibitors) that results in a tuned cargo delivery from the nanocarrier. The nanodevice displayed an enhanced cargo delivery in cancer cells (safranin O and doxorubicin) in the presence of acetylthiocholine.

Tripodal Amine Ligands for Accelerating Cu-Catalyzed Azide-Alkyne Cycloaddition: Efficiency and Stability against Oxidation and Dissociation.

Ancillary ligands, especially the tripodal ligands such as tris(triazolylmethyl)amines, have been widely used to accelerate the Cu-catalyzed azide-alkyne cycloaddition (CuAAC, a "click" reaction). However, the relationship between the activity of these Cu(I) complexes and their stability against air oxidation and ligand dissociation/exchange was seldom studied, which is critical for the applications of CuAAC in many biological systems. In this work, we synthesized twenty-one Cu(I) tripodal ligands varying in chelate arm length (five to seven atoms), donor groups (triazolyl, pyridyl and phenyl), and steric hindrance. The effects of these variables on the CuAAC reaction, air oxidation, and ligand dissociation were evaluated. Reducing the chelate arm length to five atoms, decreasing steric hindrance, or using a relatively weakly-binding ligand can significantly increase the CuAAC reactivity of the Cu(I) complexes, but the concomitant higher degree of oxidation cannot be avoided, which leads to rapid degradation of a histidine-containing peptide as a model of proteins. The oxidation of the peptide can be reduced by attaching oligo(ethylene glycol) chains to the ligands as sacrificing reagents. Using electrospray ionization mass spectrometry (ESI-MS), we directly observed the tri- and di-copper(I)-acetylide complexes in CuAAC reaction in the [5,5,5] ligand system and a small amount of di-Cu(I)-acetylide in the [5,5,6] ligand system. Only the mono-Cu(I) ligand adducts were observed in the [6,6,6] and [5,6,6] ligand systems.

Kinetic control in the temperature-dependent sequential growth of surface-confined supramolecular copolymers.

We report the sequential growth of supramolecular copolymers on gold surfaces, using oppositely charged dendritic peptide amphiphiles. By including water-solubilising thermoresponsive chains in the monomer design, we observed non-linear effects in the temperature-dependent sequential growth. The step-wise copolymerisation process is characterised using temperature dependent SPR and QCM-D measurements. At higher temperatures, dehydration of peripheral oligoethylene glycol chains supports copolymer growth due to more favourable comonomer interactions. Both monomers incorporate methionine amino acids but remarkably, desorption of the copolymers via competing sulphur gold interactions with the initial monomer layer is not observed. The surface-confined supramolecular copolymers remain kinetically trapped on the metal surface at near neutral pH and form viscoelastic films with a tuneable thickness.

Cooperative nanoparticle H-type self-assembly of a bolaamphiphilic BODIPY derivative in aqueous medium

We report the H-type self-aggregation of a new linear water-soluble BODIPY bolaamphiphile equipped with amide groups and oligoethylene glycol chains. Intermolecular π-π interactions, amide-based hydrogen bonds and hydrophobic interactions lead to a highly cooperative self-assembly process. Detailed spectroscopic studies suggest that strong hydrophobic interactions are the driving force for a rotational displacement of the dye molecules within the stack in order to shield the hydrophobic π-system from water molecules. The so formed highly-ordered micellar structures have been further characterized by NMR, AFM, TEM, DLS studies and theoretical calculations.

A Family of Amphiphilic Cyclodextrin Liquid Crystals Governed by Dipole-Dipole Interactions.

A novel family of amphiphilic cyclodextrin(CD)-based liquid crystals that bear O-acetylated oligoethylene glycol chains at the secondary face is reported. Unlike most of the previously reported liquid crystals (LC) based on chemically modified CDs, which depend on H-bonding as the primary intermolecular forces, the present CD derivatives self-assemble into highly ordered smectic liquid crystal phases via the weaker dipole-dipole intermolecular interactions. The obtained materials are found to display much improved properties such as improved thermostability, reduced clearing temperatures, and better fluidity. The present work opens up new possibilities to design CD-based LC materials.

Thermal- and pH-Dependent Size Variable Radical Nanoparticles and Its Water Proton Relaxivity for Metal-Free MRI Functional Contrast Agents.

For development of the metal-free MRI contrast agents, we prepared the supra-molecular organic radical, TEMPO-UBD, carrying TEMPO radical, as well as the urea, alkyl group, and phenyl ring, which demonstrate self-assembly behaviors using noncovalent bonds in an aqueous solution. In addition, TEMPO-UBD has the tertiary amine and the oligoethylene glycol chains (OEGs) for the function of pH and thermal responsiveness. By dynamic light scattering and transmission electron microscopy imaging, the resulting self-assembly was seen to form the spherical nanoparticles 10-150 nm in size. On heating, interestingly, the nanoparticles showed a lower critical solution temperature (LCST) behavior having two-step variation. This double-LCST behavior is the first such example among the supra-molecules. To evaluate of the ability as MRI contrast agents, the values of proton ((1)H) longitudinal relaxivity (r1) were determined using MRI apparatus. In conditions below and above CAC at pH 7.0, the distinguishable r1 values were estimated to be 0.17 and 0.21 mM(-1) s(1), indicating the suppression of fast tumbling motion of TEMPO moiety in a nanoparticle. Furthermore, r1 values became larger in the order of pH 7.0 > 9.0 > 5.0. Those thermal and pH dependencies indicated the possibility of metal-fee MRI functional contrast agents in the future.

Entropically Driven Self‐Assembly of Bolaamphiphilic Perylene Dyes in Water

AbstractThe specific hydrophobic effect involved in the self‐assembly of a bolaamphiphilic perylene bisimide (PBI) dye bearing oligoethylene glycol (OEG) chains has been identified. In pure water, the self‐assembly is entropically driven and enthalpically disfavored, as explored by optical spectroscopy and isothermal titration calorimetry studies. Besides strong π–π interactions between the PBI units that are primarily of enthalpic nature, the major contribution to the self‐assembly is the gain of entropy by release of confined water molecules from the hydration shell of the hydrophilic OEG moieties. Both contributions favor self‐assembly, but their countervailing thermodynamic parameters are reflected in an uncommon temperature dependence, which can be inverted upon gradual addition of an organic cosolvent that makes the π–π interaction increasingly dominant.

Entropically Driven Self-Assembly of Bolaamphiphilic Perylene Dyes in Water.

The specific hydrophobic effect involved in the self-assembly of a bolaamphiphilic perylene bisimide (PBI) dye bearing oligoethylene glycol (OEG) chains has been identified. In pure water, the self-assembly is entropically driven and enthalpically disfavored, as explored by optical spectroscopy and isothermal titration calorimetry studies. Besides strong π-π interactions between the PBI units that are primarily of enthalpic nature, the major contribution to the self-assembly is the gain of entropy by release of confined water molecules from the hydration shell of the hydrophilic OEG moieties. Both contributions favor self-assembly, but their countervailing thermodynamic parameters are reflected in an uncommon temperature dependence, which can be inverted upon gradual addition of an organic cosolvent that makes the π-π interaction increasingly dominant.